Process for the production of large denier carbon fibers

ABSTRACT

An improved process is provided for the expeditious formation of large denier carbon fibers, i.e. carbon fibers having a single filament denier of at least 30. A fibrous polybenzimidazole starting material of relatively large denier is initially converted to a polybenzimidazonium salt by contact with a solution of certain acids at an elevated temperature, and the resulting fibrous material sequentially is heated in oxygencontaining and non-oxidizing gaseous atmospheres at successively elevated temperatures. The resulting carbonaceous fibrous material contains at least 90 percent carbon by weight and particularly is suited for use as a reinforcing medium in a matrix material, e.g. either a polymeric or metallic matrix material.

United States Patent [191 Kalnin et al.

[ 1 Sept. 2, 1975 PROCESS FOR THE PRODUCTION OF LARGE DENIER CARBONFIBERS [73] Assignee: Celanese Corporation, New York,

[22] Filed: Apr. 15, 1974 [21] Appl. No.: 461,201

[52] US. Cl. 423/447; 264/29 [51] Int. Cl. COlb 31/07 [58] Field ofSearch 423/447; 264/29 [56] References Cited UNITED STATES PATENTS3,305,315 2/1967 Bacon et al 423/447 3,449,077 6/1969 Stuetz 423/4473,528,774 9/1970 Ezekiel et a1. 423/447 3,595,946 7/1971 .100 et al.423/447 X 3,634,035 1/1972 Stuetz 423/447 3,635,675 l/l972 Ezekiel423/447 3,656,910 4/1972 Ferment 423/447 X 3,666,417 5/1972 Araki et al.423/447 3,720,759 3/1973 Overhoff 423/447 FOREIGN PATENTS ORAPPLICATIONS 47-26974 7/1972 Japan 423/447 Primary Examiner-Edward .I.Meros 5 7 ABSTRACT An improved process is provided for the expeditiousformation of large denier carbon fibers, i.e. carbon fibers having asingle filament denier of at least 30. A fibrous polybenzimidazolestarting material of relatively large denier is initially converted to apolybenzimidazonium salt by contact with a solution of certain acids atan elevated temperature, and the resulting fibrous material sequentiallyis heated in oxygencontaining and non-oxidizing gaseous atmospheres atsuccessively elevated temperatures. The resulting carbonaceous fibrousmaterial contains at least 90 percent carbon by weight and particularlyis suited for use as a reinforcing medium in a matrix material, e.g.either a polymeric or metallic matrix material.

18 Claims, N0 Drawings PROEESS FOR THE PRODUCTION OF LARGE DENIER CARBONFIBERS BACKGROUND OF THE INVENTION In the search for high performancematerials, considerable interest has been focused upon carbon fibers.The term carbon fibers is used herein in its generic sense and includesgraphite fibers as well as amorphous carbon fibers. Graphite fibers aredefined herein as fibers which consist essentially of carbon and have apredominant x-ray diffraction pattern characteristic of graphite.Amorphous carbon fibers on the other hand, are defined as fibers inwhich the bulk of the fiber weight can be attributed to carbon and whichexhibit an essentially amorphous x-ray diffraction pattern. Graphitefibers generally have a higher Youngs modulus than do amorphous carbonfibers and in addition are more highly electrically and thermallyconductive.

As is well known to those skilled in the art, carbon fibers commonlyhave been formed by the thermal stabilization of a variety of polymericfibrous materials, and the subsequent carbonization or carbonization andgraphitization of the same in an inert atmosphere. Representative U.S.patents disclosing the production of carbon fibers from an acrylicfibrous precursor include: U.S. Pat. Nos. 2,913,802; 3,285,696;3,539,295; 3,592,595; 3,647,770; 3,650,668; 3,656,882; 3,656,883;3,708,326; and 3,729,549. Representative U.S. patents disclosing theproduction of carbon fibers from a polybenzimidazole fibrous materialinclude: U.S. Pat. Nos. 3,449,077; 3,528,774; 3,634,035; and 3,635,675.

Heretofore, the carbon fibers produced in the prior art have tended tohave a relatively small denier per filament, e.g. about 0.5 to 2.0denier per filament corresponding to an average filament diameter ofabout 0.0003 to 0.0005 inches. Whenever attempts have been made toproduce carbon fibers of relative large denier per filament, e.g. 30 to400 or more, and a filament diameter of 0.00] inch or more specialproblems have been presented particularly during the stabilizationportion of the process. It has been recognized that unless the polymericfibrous precursor is adequately stabilized (e.g. by heating in air orother oxidizing atmosphere), it cannot be satisfactorily carbonized orcarbonized and graphitized. During the stabilization portion of theprocess it has been found that an oxidized surface layer tends initiallyto form upon the fiber surface which tends to impede oxygen diffusioninto the fiber and to retard the further stabilization thereof.Accordingly extremely long stabilization periods have been required whenthe fibrous precursor is a relatively large denier per filament.Additionally, the subsequent carbonization treatment of the resultingstabilized fibrous material has tended to be slow.

Commonly assigned U.S. Ser. No. 296,725, filed Oct. ll, 1972, nowabandoned, of John W. Soehngen discloses an approach for lessening thetime required for the stabilization of an acrylic fibrous precursor oflarger than usual diameter.

Industrial high performance materials of the future are projected tomake substantial utilization of fiber reinforced composites whereincarbon fibers are incorporated in a resinous or metallic matrix. Carbonfibers these desirable properties are corrosion and high temperatureresistance, low density, high tensile strength, and high modulus.Graphite is one of the very few known materials whose tensile strengthincreases with temperature. Uses for carbon fiber reinforced compositesinclude recreational equipment, such as golf club shafts, aerospacestructural components, rocket motor casings, deep-submergence vessels,ablative materials for heat shields on re-entry vehicles, etc.

There has remained a need for improved techniques to produce carbonfibers of a relatively large denier which are derived from a polymericfibrous material of relatively large denier. Such large denier carbonfibers are capable, inter alia, of forming composite articles exhibitingan enhanced compressive strength to tensile strength ratio. Also, suchlarge denier carbon fibers are particularly suited for use as fibrousreinforcement in a metallic matrix. When one attempts to incorporate arelatively small diameter carbon filament in a metallic matrix, it hasbeen observed that the outer portion of the filament tends to react withthe matrix during composite fabrication which results in a loss of mostof the filament strength. Accordingly, commercially practicableprocesses for producing large denier carbon filaments are in demand, buthave proven to be an elusive goal.

It is an object of the invention to provide an improved process for theproduction of large denier carbon fibers.

It is an object of the invention to provide an improved process for theproduction of large denier carbon fibers beginning with large denierpolybenzimidazole fibrous precursors.

It is an object of the invention to provide a process for the productionof large denier carbon fibers beginning with a large denierpolybenzimidazole fibrous precursor wherein the stabilization portionthereof is carried out on an expeditious basis without the necessity toemploy extremely long residence times as commonly required in the priorart thereby yielding improved production efficiency.

It is another object of the invention to provide large denier carbonfilaments which may be readily substituted for boron filaments asreinforcement in a metallic matrix.

It is another object of the invention to provide large denier carbonfilaments which may be used as a substrate for receiving the vapordeposition of boron to form a boron-carbon composite fiber suitable forincorporation in a metallic matrix.

It is a further object of the invention to provide an improved processfor the production of large denier carbon fibers in which the variousthermal processing steps thereof expeditiously may be carried out in aninline continuous manner.

These and other objects, as well as, the scope, nature, and utilizationof the process will be apparent to those skilled in the art from thefollowing description and appended claims.

SUMMARY OF THE INVENTION It has been found that an improved process forthe formation of a large denier carbonaceous fibrous material comprises:

a. contacting a polybenzimidazole fibrous material having a denier perfilament of about 50 to 600 with a solution of an acid having a pK valuebelow about 4.5 while at an elevated temperature to transform saidpolybenzimidazole to a polybenzimidazonium salt wherein the anion of thesalt is derived from the acid,

b. heating the fibrous material following contact with the acid in anoxygen-containing gaseous atmosphere at a temperature of about 300 to530C. to render the fibrous material capable of undergoing carbonizationwhile retaining the original fibrous configuration substantially intact,and

c. heating the resulting fibrous material in a nonoxidizing gaseousatmosphere at a temperature of at least lOOC. until a carbonaceousfibrous material is formed which contains at least 90 percent carbon byweight'and retains the original fibrous configuration substantiallyintact.

The resulting large denier carbonaceous fibrous material particularly issuited for use as a reinforcing medium in a matrix material, e.g. apolymeric or metallic matrix material.

DESCRlPTlON OF PREFERRED EMBODIMENTS The Starting Material The largedenier polybenzimidazole fibrous material which serves as the startingmaterial has a denier per filament of about 50 to 600 and an averagefilament diameter of about 0.003 to 0.010 inch. In a preferredembodiment of the process the large denier polybenzimidazole fibrousmaterial has a denier per filament of about 100 to 500 and a filamentdiameter of about 0.004 to 0.009 inch.

Polybenzimidazoles are a known class of heterocyclic polymers. Typicalpolymers of this class and their preparation are more fully described inU.S. Pat. No. 2,895,948, U.S. Pat. No. Re. 26,065, and in the Journal ofPolymer Science, Vol. 50, pages 511-539 (1961) which are hereinincorporated by reference. The polybenzimidazoles consist essentially ofrecurring units of the following Formulas l and II. Formula I is:

wherein R is a tctravalent aromatic nucleus, preferably symmetricallysubstituted, with the nitrogen atoms forming the benzimidazole ringsbeing paired upon adjacent carbon atoms, i.e. ortho carbon atoms, of thearomatic nucleus, and R is a member of the class consisting of l anaromatic ring, (2) an alkylene group (preferably those having 4 to 8carbon atoms), and (3) a heterocyclic ring from the class consisting of(a) pyridine, (b) pyrazine, (c) furan, (d) quinoline, (e) thiophene, and(f) pyran.

Formula II is:

wherein Z ,is an aromatic nucleus having the nitrogen atoms forming thebenzimidazole ring paired upon adjacent carbon atoms of the aromaticnucleus.

Preferably, aromatic polybenzimidazoles are selected, e.g., polymersconsisting essentially of the recurring units of Formulas l and IIwherein R is at least one aromatic ring or a heterocyclic ring.

As set forth in U.S. Pat. No. Re. 26,065, the aromatic a N cpolybenzimidazoles having the recurring units of Formula [I may beprepared by self-condensing a trifunctional aromatic compound containingonly a single set of ortho disposed diamino substituents and anaromatic, preferably phenyl, carboxylate ester substituent. Exemplary ofpolymers of this type is poly-2,5(6)- benzimidazole prepared by theautocondensation of phenyl-3,4-diaminobenzoate.

As also set forth in the above-mentioned patent, the aromaticpolybenzimidazoles having the recurring units of Formula I may beprepared by condensing an aromatic tetraamine compound containing a pairof orthodiamino substituents on the aromatic nucleus with a dicarboxylcompound selected from the class consisting of (a) the diphenyl ester ofan aromatic dicarboxylic acid, (b) the diphenyl ester of a heterocyclicdicarboxylic acid wherein the carboxyl groups are substituents upon acarbon in a ring compound selected from the class consisting ofpyridine, pyrazine, furan, quinoline, thiophene and pyran and (c) ananhydride of an aromatic dicarboxylic acid.

Examples of polybenzimidazoles which have the recurring structure ofFormula I are as follows:

poly-2,2 m-phenylene )-5 ,5 bibenzimidazole;

poly-2,2 '-(pyridylene-3 ',5 )-5 ,5 '-bibenzimidazole;

poly-2,2 -(furylene-2 ',5 )-5 ,5 '-bibenzimidazole;

poly-2,2-( naphthalene-l ',6 )-5,5

bibenzimidazole;

poly-2,2-(biphenylene-4",4")-5,5-

bibenzimidazole;

'poly-2,2-amylene-5 ,5 '-bibenzimidazole;

poly-2,2 '-octamethylene-5 ,5 -bibenzimidazole;

poly-2,6-(m-phenylene)-diimidazobenzene;

poly-2 ,2 -cyclohexeneyl-5 ,5 '-bibenzimidazole;

poly-2,2 -(m-phenylene )-5 ,5 '-di( benzimidazole) ether;

poly2,2 m-phenylene )-5 ,5 '-di( benzimidazole) sulfide:poly-2,2'(m-phenylene)-5,5'-di(benzimidazole) sulfone;

poly-2,2 m-phenylene)-5 ,5 -di( benzimidazole) methane; poly-2 ',2m-phenylene )-5 ',5 '-di( benzimidazole propane-2,2; and

poly-2 ',2' '-(m-phenylene )-5 ',5 -di( benzimidazole)ethylenel ,2 wherethe double bonds of the ethylene groups are intact in the final polymer.

The preferred polybenzimidazole for use in the present process is oneprepared from poly-2,2'-( mphenylene)-5,5-bibenzimidazole, the recurringunit of which is:

Any polymerization process known to those skilled in the art may beemployed to prepare the polyben' zimidazole which may then be formedinto a continuous length of fibrous material. Representative techniquesfor preparing the polybenzimidazole are disclosed in US. Pat. Nos.3,509,108; 3,549,603; and 3,551,389, which are assigned to the assigneeof the present invention and are herein incorporated by reference.

With respect to aromatic polybenzimidazoles, preferably equimolarquantities of the monomeric tetraamine and dicarboxyl compound areintroduced into a first stage melt polymerization reaction zone andheated therein at a temperature above about 200C., preferably at least250C, and more preferably from about 270 to 300C. The reaction isconducted in a substantially oxygen-free atmosphere, i.e., below aboutppm oxygen and preferably below about 8 ppm oxygen, until a foamedprepolymer is formed having an inherent viscosity, expressed asdeciliters per gram, of at least 0.1 and preferably from about 0.13 to0.3, the inherent viscosity (I.V.) as used herein being determined froma solution of 0.4 grams of the polymer in 100 ml. of 97 percent H 50 atC.

After the conclusion of the first stage reaction, which normally takesat least 0.5 hour and preferably 1 to 3 hours, the foamed prepolymer iscooled and then powdered or pulverized in any convenient manner. Theresulting prepolymer powder is then introduced into a second stagepolymerization reaction zone wherein it is heated under substantiallyoxygen-free conditions, as described above, to yield a polybenzimidazolepolymer product, desirably having an l.V., as measured above, of atleast 0.6, e.g., 0.80 to 1.1 or more.

The temperature employed in the second stage is at least 250C,preferably at least 325C, and more pref erably from about 350 to 425C.The second stage reaction generally takes at least 0.5 hour, andpreferably from about 1 to 4 hours or more.

A particularly preferred method for preparing the polybenzimidazole isdisclosed in the aforesaid US. Pat. No. 3,509,108. As disclosed thereinaromatic poly benzimidazoles may be prepared by initially reacting themonomer in a melt phase polymerization at a temperature above about200C. and a pressure above 50 psi (e.g., 300 to 600 psi) and thenheating the resulting reaction product in a solid state polymerizationat a temperature above about 300C. (e.g. 350 to 500C.) to yield thefinal product.

The term polybenzimidazole fibrous material" as used herein includesmonofilaments, as well as multifilament fibrous materials, such as yarn,strand, cable, tow, or other continuous or discontinuous fibrousassemblage. In a preferred embodiment of the process thepolybenzimidazole fibrous material is a multifilament yarn or amultifilament tow.

As is known in the art, polybenzimidazoles are generally formed intocontinuous lengths of fibrous materials by solution spinning, that is,by dry or wet spinning a solution of the polymer in an appropriatesolvent such as N,N-dimethylacetamide, N,N-dimethylformamide,dimethylsulfoxide or sulfuric acid (used only in wet spinning) throughan opening of predetermined shape into an evaporative atmosphere for thesolvent in which most of the solvent is evaporated (dry) or into acoagulation bath (wet), resulting in the polymer having the 4' desiredfilamentary shape.

The polymer solutions may be prepared in accordance with knownprocedures. For example, sufficient polybenzimidazole may be dissolvedin the solvent to yield a final solution suitable for extrusioncontaining from about 10 to 45 percent by weight of the polymer,

based on the total weight of the solution, preferably from about 20 to30 percent by weight.

One suitable means for dissolving the polymer in the solvent is bymixing the materials at a temperature above the atmospheric boilingpoint of the solvent, for example 25 to 120C. above such boiling pointand at a pressure of 2 to 15 atmospheres for a period of l to 5 hours.

Preferably, the polymer solutions, after suitable filtration to removeany undissolved portions, are dry spun. For example, the solutions maybe extruded through a spinneret into a conventional type downdraftspinning column containing a circulating inert gas such as nitrogen,noble gasses, combustion gasses, or superheated steam. Conveniently, thespinneret face is at a temperature of from about to 170C., the top ofthe column from about to 220C, the middle of the column from about to250C, and the bottom of the column from about to 320C. After leaving thespinning column, the continuous filamentary materials are taken up, forexample, at a speed within the range of about 50 to 350 meters or moreper minute. If the continuous filamentary materials are to be washedwhile wound on bobbins, the resulting asspun materials may be subjectedto a slight steam drawing treatment at a draw ratio of from about 1.05:1to 1.5: l in order to prevent the fibers from relaxing and falling offthe bobbin during the subsequent washing step. Further details withrespect to a method for dryspinning a continuous length of apolybenzimidazole fibrous material are shown in US. Pat. No. 3,502,756to Bohrer et al. which is assigned to the same assignee as the presentinvention and is herein incorporated by reference.

The continuous length of polybenzimidazole fibrous material is nextwashed so as to remove at least the major portion of residual spinningsolvent, e.g., so that the washed materials contain less than about 1percent by weight solvent based on the weight of the continuousfilamentary material, and preferably so as to obtain an essentiallyspinning solvent-free fibrous material (i.e., a fibrous materialcontaining less than about 0.1 percent solvent by weight). Typically, asimple water wash is employed; however, if desired, other wash materials such as acetone, methanol, methylethyl ketone and similarsolvent-miscible and volatile organic solvents may be used in place ofor in combination with the water. The washing operation may be conductedby collecting the polybenzimidazole fibrous material on perforated rollsor bobbins, immersing the rolls in the liquid wash bath and pressurewashing the fibrous material, for example, for about 2 to 48 hours ormore. Alternatively, the continuous length of polybenzimidazole fibrousmaterial may be washed on a continuous basis by passing the fibrousmaterial in the direction of its length through one or more liquid washbaths (e.g., for 1 to 10 minutes). Any wash technique known to thoseskilled in the art may be selected.

The continuous length of polybenzimidazole fibrous material may next bedried to remove the liquid wash bath by any convenient technique. Forinstance, the drying operation for bobbins of yarn may be conducted at atemperature of about 150 to 300C. for about 2 to 100 hours or more.Alternatively, the continuous length of polybenzimidazole fibrousmaterial may be dried on a continuous basis by passing the fibrousmaterial in the direction of its length through an appropriate dryingzone (e.g., an oven provided at 300 to 400C. for l to 2 minutes). Ifdrying is employed, preferably the drying temperature does not exceedabout 250C. for several hours or 400C. for more than 1 minute, as abovethese limits degradation of the fiber may occur.

The polybenzimidazole fibrous material preferably next is hot drawn at adraw ratio of about 2:1 to 5:1 in order to enhance its orientation.Representative draw procedures are disclosed in commonly assigned US.Pat. No. 3,622,660, and Ser. No. 297,511, filed Oct. 13, 1972 whichissued as US. Pat. No. 3,849,529.

The Formation of a Polybenzimidazonium Salt The large denierpolybenzimidazole fibrous material is contacted with a solution of anacid having a pK below about 4.5 (preferably below about 3.5) while atan elevated temperature to transform the polybenzimidazole to apolybenzimidazonium salt wherein the anion of the salt is derived fromthe acid.

The acid selected may be organic or inorganic in nature and preferablyis relatively non-volatile and incapable of decomposition at thetreatment temperature selected. The pK value of a given acidconveniently may be ascertained by determining the negative logarithm ofthe K for acid in a 0.1M aqueous solution at 25C. Those acids having apK value much above about 4.5 possess insufficient strength to be usefulin the production of the desired salt. Suitable acids include themineral acids, monobasic acid and dibasic carboxylic acids, and sulfonicacids.

Representative inorganic acids include: sulfamic acid, sulfuric acid,hydrochloric acid, phosphoric acid, perchloric acid, hydrobromic acid,hydrofluoric acid, hydriodic acid, etc.

Representative carboxylic acids include: acetic acid, oxalic acid,monochloroacetic acid, dichloroacetic acid, trichloroacetic acid,monofluoroacetic acid, difluoroacetic acid, trifluoroacetic acid,substituted benzoic acids, salicylic acid, etc.

Representative sulfonic acids include: benzene sulfonic acid, 6-toluenesulfonic acid, m-toluene sulfonic acid, p-toluene sulfonic acid,2,4-xylene sulfonic acid, toluene-2,4-disulfonic acid, 2-naphthalenesulfonic acid, bisphenol disulfonic acid, chlorosulfonic acid, methanesulfonic acid, trifluoromethane sulfonic acid, etc.

The particularly preferred acids for use in the process are sulfamicacid, phosphoric acid, sulfuric acid, hydrochloric acid and acetic acid.

The solvent utilized to form the solution of the acid preferably isaqueous in nature; however, other solvents such as N-propanol,ethyleneglycolmonomethyl ether, methylene chloride, methanol, etc., mayalternatively be employed.

The acid preferably may be provided in the solvent in a concentration ofabout 1 to 10 percent by weight based upon the total weight of thesolution, and most preferably in a concentration of about 2 to 5 percentby weight. The acid solution preferably is provided in a quantity suchthat its weight exceeds that of the polybenzimidazole fibrous materialundergoing treatment by about 10 to 40 times. Also the acid preferablyis provided in a quantity such that at least 1 equivalent of acid (e.g.l to 2 equivalents of acid) reacts with each repeat unit of the polymerto form the polybenzimidazonium salt. An aqueous solution of the acidoptionally may include a swelling agent for the polybenzimidazoledissolved or dispersed therein in order to aid in the uniform productionof the polybenzimidazonium salt throughout the fibrous material.Representative swelling agents include: benzyl alcohol,2-phenoxyethanol, or other partially soluble solvents having asolubility parameter in water between 1 l and 13. The swelling agentpreferably is provided in a concentration of about 3 to 20 percent byweight based upon the total weight of the solution, and most preferablyin a concentration of about 5 to 10 percent by weight. The particularlypreferred swelling agent for use in the process is benzyl alcohol.

The solution of the acid preferably is provided at a temperature ofabout 50 to 100C. (e.g. about to 98C.) when contacted with thepolybenzimidazole fibrous material. It is recommended that the fibrousmaterial be immersed in the solution of the acid in such a manner thatdirect contact with the solution throughout the fibrous material ismaximized. For instance, a continuous length of the fibrous materialwhile wound upon a frame or support to a limited thickness may bepositioned in the solution. Alternatively, a continuous length of thefibrous material may be continuously passed through the solution in thedirection of its length while substantially suspended therein. Suitableresidence times for the formation of the polybenzimidazonium saltcommonly range from about 2 to 50 minutes (e.g. 20 to 40 minutes)whilein contact with the solution of acid. Longer residence times may beselected without commensurate advantage.

Alternatively, the polybenzimidazonium salt may be formed by contact ofthe swollen as-spun polybenzimidazole fibrous material with the solutionof acid as described.

After the acid treatment the fibrous material preferably is washed,(i.e. rinsed) with water to remove excess acid, and is dried (e.g. at100 to 200C. for 15 minutes in a circulating air oven).

The formation of the polybenzimidazonium salt surprisingly has beenfound to render the fibrous material capable of undergoing stabilizationin an oxidizing atmosphere on a more expeditious basis therebyeffectively overcoming stabilization difficulties commonly associatedwith large denier polybenzimidazole fibrous materials.

The formation of a polybenzimidazonium sulfate salt upon reaction ofpoly-2,2'-(m-phenylene)-5,5'- bibenzimidazole with sulfuric acid isillustrative of the salt formation reaction and can be represented bythe following equation:

As indicated previously, it is not essential that 2 equivalents of acidreact with each repeat unit of the polymer to form thepolybenzimidazonium salt.

Once the salt formation reaction is complete the fibrous materialcontinues to exhibit its original fibrous configuration, but exhibitssubstantially different properties. For instance, the tendency for thefibrous material to shrink in length when heated in an unrestrainedstate in an incandescent flame at about 500C. is commonly reduced fromabout 80 percent to 4 or 5 percent. Shifts in uv absorption maximacommonly are observed, e.g. when sulfuric acid is used to form the salta thin polymer film on quartz may exhibit a value of 252,340 nm whilethe control of untreated poly-2,2 (m-phenylene)-5,5-bibenzimidazole filmexhibits a value of 258,357 nm. Solubility changes and thermal stabilitychanges may be observed.

Density changes may be observed. For instance when fibers of poly-2,2'-(m-phenylene )-5,5 bibenzimidazole are treated in aqueous solutions of3 percent acid and 6 percent benzyl alcohol swelling agent for 40minutes at 95C. the following densities were recorded for the resultingfibers.

Acid Utilized Density of Fiber (gm./c.c.)

Sulfuric acid Phosphoric acid Sulfamic acid Hydrochloric acid P-toluencsulfonic acid Acetic acid Trifluoroacctic acid Oxalic acid Salicylicacid Untreated control Control subjected to benzyl alcohol and wateronly Also, crystallinity changes may be apparent. Poly2,2-

-- (m-phenylene)-5,5-bibenzimidazole fibers generally Acid Utilized {A}[B] Sulfuric acid l6 l9 Hydrofluoric acid l6 l5 Phosphoric acid 20 18Perchloric acid 22 32 Sulfamic acid [9 19 The Thermal Stabilization Thefibrous material following the formation of the polybenzimidazonium saltis heated for a relatively brief residence time in a molecularoxygencontaining gaseous atmosphere at a temperature of about 300 to530C. to oxidize the fibers and to render the same capable of undergoingcarbonization while retaining the original fibrous configurationsubstantially intact. The preferred oxygen-containing gaseous atmosphereis air; however, other gaseous atmospheres containing a greater orlesser concentration of oxygen produce equally satisfactory results.

The stabilization treatment may be carried out on either a batch or acontinuous basis with the large denier fibrous material being either (I)statically positioned within the stabilization zone, or (2) continuouslypassed through the stabilization zone in the direction of its length.When the process is carried out on a batch basis, a continuous length ofthe fibrous material may be wound upon a support (e.g. a stainless steelbobbin) and placed in the stabilization zone. The stabilizationtreatment is preferably carried out while the fibrous material ismaintained at a substantially constant length.

Suitable residence times for the stabilization reaction commonly rangefrom about 1 to 30 minutes. Longer residence times may be utilizedwithout commensurate v advantage.

The period of time required to complete the stabilization reactionwithin the gaseous atmosphere generally is inversely related to thetemperature of the gaseous atmosphere, and also is influenced to somedegree by the denier of the fibrous material. During the stabilizationreaction it may be desirable that the fibrous material be graduallyraised to the maximum stabilization temperature if the resulting productis to exhibit optimum physical properties. A representative heatingprofile which is particularly advantageous when the polybenzimidazoniumsalt was formed with the aid of sulfuric acid, or sulfamic acid, is asfollows: heat at 300C. for minutes, at 400C. for 10 minutes, and at465C. for 5 minutes. When the polybenzimidazonium salt is formed withthe aid of an acid such as phosphoric acid or acetic acid, the entirestabilization reaction may be carried out at a relatively constanttemperature of about 465C. for 5 minutes. The exact stabilizationheating conditions for optimum results within the range of about 300 to530C. may be determined by simple experimentation.

The stabilized fibrous material formed in accordance with the presentprocess is black in appearance which is usually accompanied by a purpletinge, retains it original fibrous configuration substantially intact,and is capable of undergoing carbonization when heated in an inertgaseous atmosphere (e.g. at a temperature of 1000C.) without loss of itsconfiguration (e.g. through coalescence or melting). Also, the fiber canbe tensioned upon continuous carbonization in the absence of breakage.Additionally the stabilized fibrous material commonly exhibits a boundoxygen content of about 2-8 percent by weight as determined by theUnterzaucher, or other suitable analysis.

The theory whereby the initial conversion of the large denierpolybenzimidazole fibrous material to a polybenzimidazonium salt iscapable of expediting the subsequent stabilization reaction so that thedesired stabilization can be accomplished in minutes rather than hoursis considered complex and incapable of simple explanation when comparedwith the residence times required in the prior art for the same fibers.The results achieved are considered to be surprising and unexpected.

The Formation of a Large Denier Carbon Fiber The resulting stabilizedfibrous material is heated in a non-oxidizing gaseous atmosphere at atemperature of at least 1000C. until a carbonaceous fibrous material isformed which contains at least 90 percent carbon by weight (preferablyat least 95 percent carbon by weight) and retains the original fibrousconfiguration substantially intact. Carbonization or carbonization andgraphitization may be accomplished in accordance with conventionaltechniques, e.g. the utilization of induction furnaces, resistanceheated furnaces, or reducing flames as disclosed in commonly assignedUS. Pat. No. 3,449,077.

In a preferred embodiment of the process the nonoxidizing gaseousatmosphere is an inert gaseous atmosphere selected from the groupconsisting of nitrogen, argon and helium. The particularly preferredgaseous atmosphere is nitrogen.

The higher the temperature of the non-oxidizing gaseous atmosphere thegreater the degree of graphitic carbon formed within the fiber and thegreater the Youngs modulus of the fiber. Temperature profiles may beutilized wherein the fiber is heated in a nonoxidizing gaseousatmosphere having a temperature up to about 3000C. Residence times at atemperature of at least lO00C. commonly range from about 2 to 20minutes. Lesser residence times may be utilized if the resultingstabilized fibrous material is heated in a reducing flame.

The carbonization or carbonization and graphitization may be carried outon a batch or continuous basis. Since comparable residence times arerequired for the stabilization treatment and the carbonizationtreatment, these steps of the process optionally may be carried out intandem with a continous length of fibrous material being passed in thedirection of its length through the appropriate heating zones.

Commonly the carbon fibers formed in the present process have a denierper filament of about 30 to400 (e.g. to 350), and an average diameter ofabout 0.002 to 0.008 inch. The diameter of the carbon fiber product islargely determined by the diameter of the starting material but isgenerally less than that of the starting material due to loss ofnon-carbon atoms during the carbonization treatment and possiblestretching during stabilization and carbonization.

The large denier carbon fibers formed in the present process. may beincorporated in a matrix material (e.g. a polymeric or metallic matrix)to form a composite article having an enhanced compressive strength totensile strength ratio. The large denier carbon fibers are particularlysuited for incorporation in a metallic matrix, e.g. a matrix ofmanganese, aluminum, or copper. If desired, a protective coating, suchas silicon or chromium, may be applied to the fibers prior toincorporation in a metallic matrix rather than incorporating the fibersdirectly in the matrix. Also, the large denier carbon fibers may besubstituted for a tungsten fiber prior to the application of a coatingboron to yield a fiber suitable for incorporation in a composite articlehaving a lesser density than a common boron fiber.

The following examples are given as specific illustrations of theinvention. It should be understood, however, that the invention is notlimited to the specific details set forth in the examples.

EXAMPLE I A polybenzimidazole monofilament, namely poly-2,2-(m-phenylene)-5,5'-bibenzimidazole, is selected as the examplarypolybenzimidazole for use in carrying out the process of this invention.The monofilament has a denier per filament of 400 and a fiber diameterof 0.008 inch. The monofilament is formed in accordance with theprocedure described in commonly assigned US. Pat. No. 3,526,693 ofR. N.Rulison and J. P. Riggs and has been hot drawn at a draw ratio of 25:1.

The monofilament was wrapped about a mandrel'and immersed for 30 minutesin a solution consisting of 2 percent by Weight sulfuric acid, 6 percentby weight benzyl alcohol swelling agent, 0.06 percent by weight of ananionic surfactant (i.e. a sodium salt of a complex phosphate ester soldunder the designation GAFAC MC-470 surfactant), and 91.94 percent byweight water provided at a temperature of 95C. to form apolybenzimidazonium salt having a sulfate anion. The monofilament isnext washed in water to remove excess acid and is dried in air at 200C.for 15 minutes. The formation of the salt is evidenced by changes insolubility, fiber color, density, uv absorption, crystallinity and thepresence of 5.3 percent by weight sulfur therein as determined by x-rayfluorescence. Following the salt formation reaction the monofilamentretains its original fibrous configuration intact and exhibits abrighter appearance.

The monofilament while wrapped on a suitable mandrel is thermallystabilized by heating in a circulating air oven in accordance with thefollowing heating schedule: minutes at 300C, 10 minutes at 400C. and 5minutes at 465C. The stabilized monofilament is black in appearance, andretains its original fibrous configuration intact.

The stabilized monofilament while in a holder is carbonized in a tubefurnace containing a circulating nitrogen atmosphere. The monofilamentover a period of 3 minutes gradually was inserted in the furnace whichwas preheated to 1 100C, retained therein for minutes while at llO0C.,and subsequently withdrawn. The resulting large denier carbonaceousfibrous material contains in excess of 90 percent carbon by weight,retains its original fibrous configuration intact, has a mean denier of280 and a mean fiber diameter of about 0.007 inch, a filament strengthof 61,000 psi, a break elongation of 0.42 percent, a tensile modulus of14.5 million psi, a density of 1.39 gm./c.c., and is suited for use asfibrous reinforcement in a polymeric or metallic matrix material.

EXAMPLE 11 Example I is repeated with the exception that sulfamic acidis substituted for sulfuric acid in the polybenzimidazonium saltformation reaction to form a salt having a H N-SO Oanion. Also, theprocessing conditions are modified as indicated. The thermalstabilization solely is conducted in air for 5 minutes at 465C. Duringcarbonization the fibrous material is maintained for 3 minutes at 1100C. The resulting carbon filament has a mean denier of 279, a tensilestrength of 60,000 psi, a break elongation of 0.75 percent, a tensilemodulus of 8 million psi, and a density of 1.38 gr./c.c.

EXAMPLE 111 Example I is repeated with the exception that phosphoricacid is substituted for sulfuric acid in the poly benzimidazonium saltformation reaction to form a salt having a phosphate anion. Also, theprocessing conditions are modified as indicated. The thermalstabilization solely is conducted in air for 5 minutes at 465C. Duringcarbonization the fibrous material is maintained for 2 minutes at 1100C. The resulting carbon filament has a mean denier of 255, a tensilestrength of 80,000 psi, a break elongation of 0.41 percent, a tensilemodulus of 19 million psi, and a density of 1.40 gm./c.c.

Although the invention has been described with preferred embodiments, itis to be understood that variations and modifications may be resorted toas will be apparent to those skilled in the art (e.g. the process couldbe carried out on a continuous basis, etc. Such variations andmodifications are to be considered -within the purview and scope of theclaims appended hereto.

We claim:

1. An improved process for the formation of a large denier carbonaceousfibrous material comprising:

a. contacting a polybenzimidazole fibrous material having a denier perfilament of about 50 to 600 with a solution of an acid having a pK valuebelow about 4.5 while at an elevated temperature to transform saidpolybenzimidazole to a polybenzimidazonium salt wherein the anion ofsaid salt is derived from said acid,

b. heating said fibrous material following contact with said acid in anoxygen-containing gaseous atmosphere at a temperature of about 300 to530C. to render said fibrous material capable of undergo ingcarbonization while retaining the original fibrous configurationsubstantially intact, and

c. heating said resulting fibrous material in a nonoxidizing gaseousatmosphere ata temperature of at least 1000C. until a carbonaceousfibrous material is formed which contains at least percent carbon byweight and retains the original fibrous configuration substantiallyintact. i

2. An improved process for the formation of a large denier carbonaceousfibrous material in accordance with claim 1 wherein saidpolybenzimidazole fibrous material consists essentially of recurringunits of the formula:

denier carbonaceous fibrous material in accordance with claim 1 whereinsaid polybenzimidazole fibrous material is poly-2,2'-(m-phenylene )-5 ,5bibenzimidazole.

4. An improved process for the formation of a large denier carbonaceousfibrous material in accordance with claim 1 wherein saidpolybenzimidazole fibrous material has denier per filament of about to500.

5. An improved process for the formation of a large denier carbonaceousfibrous material in accordance with claim 1 wherein said acid isselected from the group consisting essentially of sulfamic acid,phosphoric acid, sulfuric acid, hydrochloric acid, and acetic acid.

6. An improved process for the formation of a large denier carbonaceousfibrous material in accordance with claim 1 wherein said solution ofsaid acid utilized in step (a) is provided at a temperature of about 50to 100C. when contacted with said fibrous material.

7. An improved process for the formation of a large denier carbonaceousfibrous material in accordance with claim 6 wherein said contact isconducted for about 2 to 50 minutes.

8. An improved process for the formation of a large denier carbonaceousfibrous material in accordance with claim 1 wherein saidoxygen-containing gaseous atmosphere is air.

9. An improved process for the formation of a large denier carbonaceousfibrous material in accordance with claim 1 wherein said fibrousmaterial following contact with said acid is heated in saidoxygencontaining atmosphere of step (b) for about I to 30 minutes.

10. An improved process for the formation of a large denier carbonaceousfibrous material in accordance with claim I wherein said non-oxidizinggaseous atmosphere of step (c) is selected from the group consistingessentially of nitrogen, argon, and helium.

11. An improved process for the formation of a large denier carbonaceousfibrous material in accordance with claim 1 wherein said resultingfibrous material is heated in said non-oxidizing gaseous atmosphere ofstep (c) for about 2 to minutes.

12. An improved process for the formation of a large denier carbonaceousfibrous material comprising:

a. contacting a poly-2,2'-(m-phenylene)-5,5'-

bibenzimidazole fibrous material having a denier per filament of about100 to 500 with an aqueous solution of an acid selected from the groupconsisting essentially of sulfamic acid, phosphoric acid, sulfuric acid,hydrochloric acid, and acetic acid, while at a temperature of about 50to 100C. for a residence time of about 2to 50 minutes to transform saidpoly-2,2-(m-phenylene)-5,5'- bibcnzimidazole to a polybenzimidazoniumsalt wherein the anion of said salt is derived from said acid, I

b. heating said resulting fibrous material following contact with saidacid in an oxygen-containing gaseous atmosphere at a temperature ofabout300 to dergoing carbonization while retaining the original fibrousconfiguration substantially intact, and

c. heating said resulting fibrous material at a temperature of at leastl000C. in a non-oxidizing gaseous atmosphere selected from the groupconsisting essentially of nitrogen, argon, and helium for a residencetime of about 2 to 20 minutes until a carbonaceous fibrous material isformed which contains at least percent carbon by weight and retains theoriginal fibrous configuration substantially intact.

13. An improved process for the formation of a large denier carbonaceousfibrous material in accordance with claim 12 wherein said fibrousmaterial is a continuous length of a multifilament yarn.

14. An improved process for the formation of a large denier carbonaceousfibrous material in accordance with claim 12 wherein said fibrousmaterial is a continuous length of a multifilament tow.

15. An improved process for the formation of a largedenier carbonaceousfibrous material in accordance with claim 12 wherein said aqueoussolution of said acid additionally includes a swelling agent for saidfibrous material.

16. An improved process for the formation of a large denier carbonaceousfibrous material in accordance with claim 15 wherein said swelling agentis benzyl alcohol. I

17. An improved process for the formation of a large denier carbonaceousfibrous material in accordance with claim 15'whercin saidoxygen-containing gaseous atmosphere of step (b) is air.

18. An improved process for the formation of a large denier carbonaceousfibrous material in accordance with claim 15 wherein said non-oxidizinggaseous atmosphere of step (c) is nitrogen.

1. AN IMPROVED PROCESS FOR THE FORMATION OF A LARGE DENIER CARBONACEOUSFIBROUS MATERIAL COMPRISING: A. CONTACTING A POLYBENZIMDAZOLE FIBROUSMATERIAL HAVING A DENIER PER FILAMENT OF ABOUT 50 TO600 WITH A SOLUTIONOF AN ACID HAVING A PK4 VALUE BELOW ABOUT 4.5 WHILE AT AN ELEVATEDTEMPERATURE TO TRANSFORM SAID POLYBENZIMIDZOLE TO A POLYBENZIMIDAZONIUMSALT WHERIN THE ANION OF SAID SALT IS DERIVED FROM SAID ACID, B. HEATINGSAID FIBROUUS MATERIAL FOLLOWING CONTACT WITH SAID ACID IN ANOXYGEN-CONTAINING GASEOUS ATMOSPHERE AT A TEMPERATURE AT ABOUT 300* TO530*C. TO RENDER SAID FIBROUS MATERIAL CAPABLE OF UNDERGOINGCARBONIZATION WHILE RETAINING THE ORIGINAL FIBROUS CONFIGURATIONSUBSTANTIALLY INTACT, AND C. HEATING SAID RESULTING FIBROUS MATERIAL INA NON-OXIDIZING GASEOUS ATMOSPHERE AT A TEMPERATURE OF AT LEAST 1000*C.UNTIL A CARBONACEOUS FIBROUS MATERIAL IS FORMED WHICH CONTAINS AT LEAST90 PERCENT CARBON BY WEIGHT AND RETAINS THE ORGINAL FIBROUSCONFIGURATION SUBSTANTIALLY INTACT.
 2. An improved process for theformation of a large denier carbonaceous fibrous material in accordancewith claim 1 wherein said polybenzimidazole fibrous material consistsessentially of recurring units of the formula:
 3. An improved processfor the formation of a large denier carbonaceous fibrous material inaccordance with claim 1 wherein said polybenzimidazole fibrous materialis poly-2,2''-(m-phenylene)-5,5''-bibenzimidazole.
 4. An improvedprocess for the formation of a large denier carbonaceous fibrousmaterial in accordance with claim 1 wherein said polybenzimidazolefibrous material has denier per filament of about 100 to
 500. 5. Animproved process for the formation of a large denier carbonaceousfibrous material in accordance with claim 1 wherein said acid isselected from the group consisting essentially of sulfamic acid,phosphoric acid, sulfuric acid, hydrochloric acid, and acetic acid. 6.An improved process for the formation of a large denier carbonaceousfibrous material in accordance with claim 1 wherein said solution ofsaid acid utilized in step (a) is provided at a temperature of about 50*to 100*C. when contacted with said fibrous material.
 7. An improvedprocess for the formation of a large denier carbonaceous fibrousmaterial in accordance with claim 6 wherein said contact is conductedfor about 2 to 50 minutes.
 8. An improved process for the formation of alarge denier carbonaceous fibrous material in accordance with claim 1wherein said oxygen-containing gaseous atmosphere is air.
 9. An improvedprocess for the formation of a large denier carbonaceous fibrousmaterial in accordance with claim 1 wherein said fibrous materialfollowing contact with said acid is heated in said oxygen-containingatmosphere of step (b) for about 1 to 30 minutes.
 10. An improvedprocess for the formation of a large denier carbonaceous fibrousmaterial in accordance with claim 1 wherein said non-oxidizing gaseousatmosphere of step (c) is selected from the group consisting essentiallyof nitrogen, argon, and helium.
 11. An improved process for theformation of a large denier carbonaceous fibrous material in accordancewith claim 1 wherein said resulting fibrous material is heated in saidnon-oxidizing gaseous atmosphere of step (c) for about 2 to 20 minutes.12. An improved process for the formation of a large denier carbonaceousfibrous material comprising: a. contacting apoly-2,2''-(m-phenylene)-5,5''-bibenzimidazole fibrous material having adenier per filament of about 100 to 500 with an aqueous solution of anacid selected from the group consisting essentially of sulfamic acid,phosphoric acid, sulfuric acid, hydrochloric acid and acetic acid, whileat a temperature of about 50* to 100*C. for a residence time of about 2to 50 minutes to transform saidpoly-2,2''-(m-phenylene)-5,5''-bibenzimidazole to a polybenzimidazoniumsalt wherein the anion of said salt is derived from said acid, b.heating said resulting fibrous material following contact with said acidin an oxygen-containing gaseous atmosphere at a temperature of about300* to 530*C. for a residence time of about 1 to 30 minutes to rendersaid fibrous material capable of undergoing carbonization whileretaining the original fibrous configuration substantially intact, andc. heating said resulting fibrous material at a temperature of at least1000*C. in a non-oxidizing gaseous atmosphere selected from the groupconsisting essentially of nitrogen, argon, and helium for a residencetime of about 2 to 20 minutes until a carbonaceous fibrous material isformed which contains at least 90 percent carbon by weight and retainsthe original fibrous configuration substantially intacT.
 13. An improvedprocess for the formation of a large denier carbonaceous fibrousmaterial in accordance with claim 12 wherein said fibrous material is acontinuous length of a multifilament yarn.
 14. An improved process forthe formation of a large denier carbonaceous fibrous material inaccordance with claim 12 wherein said fibrous material is a continuouslength of a multifilament tow.
 15. An improved process for the formationof a large denier carbonaceous fibrous material in accordance with claim12 wherein said aqueous solution of said acid additionally includes aswelling agent for said fibrous material.
 16. An improved process forthe formation of a large denier carbonaceous fibrous material inaccordance with claim 15 wherein said swelling agent is benzyl alcohol.17. An improved process for the formation of a large denier carbonaceousfibrous material in accordance with claim 15 wherein saidoxygen-containing gaseous atmosphere of step (b) is air.
 18. An improvedprocess for the formation of a large denier carbonaceous fibrousmaterial in accordance with claim 15 wherein said non-oxidizing gaseousatmosphere of step (c) is nitrogen.